Good question, this made me think.

You get a reward for being a mining node, not just any node. Even then, do miners have much incentive to share blocks, other than the ones they mine?

I think the incentive is mutual for most nodes (aside from the mining aspect). People will set up a node to accept transactions in an automated manner, or to have higher confidence in the state of their accounts.

It's like being on the floor of the stock market. People participating want to be where all the information is (for their own benefit), and there is incentive to bring others in and share information (because it increases the amount of information you have).

I suppose you could be a "selfish" node. The bitcoin-equivalent of someone who leeches and never seeds. But the advantage is low relative to the amount of money moving around. Most people don't care about the bandwidth of running a bitcoin node, they care about latency. Unlike bittorrent, there isn't a de-facto finished version of the file being synced: it's a constantly-updating list that everyone wants to have the latest version of. I can't find the words, but this seems to be the fundamental difference.

What's the incentive for people to participate in file sharing networks? To some degree it's access to a world of free media (same as access to a world of decentralized identities), but to a large degree it's an interesting hobby/excuse to be interested in tech. Some people have racks of hard drives dedicated to hobbies like this, just because it's interesting and is worthy.

For me the incentive is being able to own an identity that nobody can take away from me. And the assumption is that services will support this type of identity, so I don't have to make accounts on other systems that people can take away and now I've lost all access to any data I had.